batman-adv: don't allow ECTP traffic on batman-adv

We have seen this to break networks when used with bridge loop
avoidance. As we can't see any benefit from sending these ancient frames
via our mesh, we just drop them.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>